This invention relates to a single-mode optical fiber which is used in a wavelength band range of 1.2 to 1.8 .mu.m for optical telecommunication.
Generally, an optical fiber of this type is produced by drawing preformed rods produced by a modified chemical vapor deposition process (MCVD) for the purpose of obtaining the fiber of low transmission loss and high transmission capacity. More particularly, an optical fiber is made by a process which has the steps of supplying raw material gas such as silcon tetrachloride, metallic chloride as a dopant for changing the refractive index, and also oxygen into a quartz glass tube rotating the tube on its tubular axis as a rotating axis from one end to the other end of the tube and applying heat from a heater moving along the tube and arranged outside the tube and heating the inside of the tube to oxidize the raw material gas so as to sequentially deposit SiO.sub.2 glass cladding and core on the inner wall of the tube, then increasing the heat from the heater to soften the tube so as to form a rod which is located in the hollow portion of the tube, and then gradually hot drawing the formed rods from the ends thereof.
The conventional single-mode optical fiber made by the aforementioned process was heretofore made for that use in a wavelength range of 0.8 to 0.9 .mu.m of semiconductor laser diode (LD) and incorporated with a core of pure SiO.sub.2 glass and a cladding of B.sub.2 O.sub.3 -SiO.sub.2 glass.
The optical fiber of such a structure can reduce Rayleigh scattering (which has a loss proportional to 1/.lambda..sup.4 where .lambda. represents the used wavelength of light) which occurred due to the irregular array of glass molecules with the SiO.sub.2 of high purity.
In order to obtain an optical fiber of low loss in longer wavelength bands of 1.2 to 1.8 .mu.m, there is the problem of the increase in loss caused by the vibration absorption in infrared B-O bond at the shorter wavelength side as compared with Si-O P-O bond. Accordingly, the optical fiber used in the above longer wavelength must not contain, the B.sub.2 O.sub.3. Thus, the conventional MCVD process employs the steps of depositing pure SiO.sub.2 glass (cladding) containing no B.sub.2 O.sub.3 on the inner wall of a quartz glass tube (as a supporting layer) and then depositing a layer to become a core. In addition, the conventional process formed a thicker cladding to thereby prevent the increase of loss of light waves of 1.38 .mu.m due to the absorption of OH ions contained in the tube into the core.
When the conventional quartz glass tube thus employs the aforementioned pure SiO.sub.2 glass as cladding, it requires a deposition temperature of 1,500.degree. to 1,600.degree. C. to form the cladding and also requires a long time to form the cladding of the same thickness due to low deposition speed. Furthermore, because of the long time and high temperature required to form the cladding, the OH ion contained in the quartz glass tube diffuses into the cladding deposited on the inner wall of the tube and even into the core upon completion of the glass fiber making it impossible to obtain an optical fiber having a low transmission loss. The high temperature further resulted in advancing the collapse of the quartz glass tube. There also occurred the problem of continuing the process while pressurizing the interior of the tube owing to the reduction in outer diameter of the tube if the process is continued with the advancement of the self-collapse of the tube.